1. Field of the Invention
The present invention relates to the information transcoding field and relates more particularly still to a process for transforming a flow of information, binary for example, into a flow of pulses better adapted to the usual transmission media (metal conductor cable, optical fiber cable, etc.); this flow is very often a succession of ternary pulses, i.e. comprising three levels.
The invention also relates to a transmission system for implementing this process.
2. Description of the Prior Art
The problem posed by such transcoding forms the subject of numerous studies, among which may be mentioned by way of example an article by J. Valin, appearing in the Thomson-CSF technical review (Volume II, No. 2, June 1979, page 359); it emerges more especially therefrom that the characteristics desired for a code are: minimization of the passband normally required for correctly transmitting the signal; suppression of the DC component, so as to allow simplification of certain elements of the transmission system; the possibility of having a self-synchronizing process so as to avoid the addition of a special channel for transmitting synchronizing signals, and intrinsic detection of transmission errors. A number of complex codes in particular are known, described more especially in the above-mentioned article, such as ternary, quaternary codes, etc., in which the number of possible combinations is greater than that of the starting code, for example binary. This redundance allows several alphabets to be defined, that is to say that for each possible value of the starting code there correspond several values of the complex code.
A coding law is the concept which allows, for each value of the starting code, that one of the alphabets which will be used to be chosen, depending at least on one criterion which takes into account the whole of the message already transmitted: this criterion is the current numerical sum, which it is recalled is the sum of the values of the symbols of the code between the time chosen as the beginning of emission and any time t. Now, it is known that limitation of the numerical sum allows more especially the DC component of the transmitted signal to be eliminated: the coding law regulates therefore in general the passage from one alphabet to another with a view to reducing the current numerical sum.